Middlesbrough Council children's services provision is "inadequate", a government watchdog has found.
Ofsted inspectors said the service had "deteriorated" since 2017 and there were "serious and widespread failures".
Children had been left too long in harmful situations, risks were not properly recognised and insufficient action was taken to help, they said.
Middlesbrough mayor Andy Preston has apologised and said there were "no excuses".
"The children and people of Middlesbrough deserve the very best and we have fallen way short of that," he said.
"We fully accept the inspectors' findings which are the culmination of a number of factors going back many years."
'Long standing concerns'
Ofsted inspectors said managers knew significant improvements were needed but did not appreciate how bad the situation was.
Action taken to reduce delays had not worked and improvements made following an inspection in 2017 found "serious cause for concern" did not go far enough, they found.
A three-week inspection in November and December found risks were not identified and responded to effectively, "particularly in relation to long-standing concerns of chronic neglect and wider exploitation".
Although some responses to significant concerns of harm were quick and effective, there were also delays, inadequate screening and inconsistent social work practice in other cases, inspectors said.
Some assessments failed to appreciate harm and lead to "over-optimistic" decision-making.
Help for children who went missing and those vulnerable to exploitation was "mostly ineffective", the Ofsted report said.
Handling of allegations against professionals working with children "had not been effective for a number of months".
Children who were in care, or had been, were "vulnerable to poor planning and instability", inspectors found.
Their report noted the town had a "particularly high" rate of children requiring care and the rate was increasing.
The council blamed government cuts, "ballooning" demand for services and inflation for the "considerable strain on the delivery of services, including those for children".
In November it emerged its children's services department had overspent its budget by more than £5m.